Unlike earlier prototypes, the new nuclear clock can maintain its own stability without depending on a traditional atomic clock. Researchers demonstrated that it can operate stably for more than 24 hours without intervention.

The achievement marks a significant advance in precision measurement, or metrology. By using the atomic nucleus as a highly stable reference, nuclear clocks could eventually measure time and other physical quantities with a level of precision beyond the capabilities of existing technology.

Why thorium is the key to nuclear clocks

The breakthrough depends on the unusual properties of thorium nuclei, which scientists have studied for decades.

Atomic nuclei can exist in different energy states, but transitions between those states usually require enormous amounts of energy. Thorium is a rare exception: two of its nuclear energy states are separated by an unusually small energy gap. That makes it possible to trigger a transition between them using laser light.

This property allows researchers to control the energy states of atomic nuclei with lasers—something that is not possible in the same way for most other nuclei. Scientists can then use this extremely precise transition as a reference for measuring time.

Key breakthroughs led to the self-stabilizing clock

A major step forward came in April 2024, when researchers led by Professor Torsten Schumm of the Institute of Atomic and Subatomic Physics at the Vienna University of Technology worked with a team led by Professor Ekkehard Peik of PTB Braunschweig.

The researchers experimentally identified the long-sought nuclear transition and demonstrated for the first time that a laser beam could excite thorium nuclei.

Further progress followed in the fall of 2024. Researchers connected a thorium-excited device to a conventional optical atomic clock and showed that atomic nuclei could serve as highly accurate timekeeping standards.

Those experiments established the basic principles of a nuclear clock. However, one important capability was still missing: the ability for the nuclear clock to maintain its own accuracy without relying on a conventional atomic clock.

How the nuclear clock stabilizes itself

“What we really want is a self-stabilizing nuclear clock,” explains Professor Torsten Schumm. “The basic idea is simple: you have a laser and thorium. The laser changes the energy state of the thorium nucleus, and the thorium nucleus is used to stabilize the frequency of the laser.”

To achieve this, the researchers developed a system based on a special crystal containing thorium atoms. The crystal was manufactured at the Vienna University of Technology. A laser shines into the crystal and interacts with the thorium nuclei inside.

Laser light oscillates at a specific frequency, creating a regular rhythm that can be used to measure time. However, even small changes in the surrounding environment can disturb that rhythm. Temperature fluctuations, for example, can cause the laser frequency to shift slightly.

“This oscillation of laser light can be used to measure time, but the laser frequency can sometimes change slightly, for example due to temperature fluctuations,” Schumm explains. “Therefore, to make high-precision measurements, we need a mechanism to keep the laser frequency precisely stable so that the clock continues to run at exactly the same rhythm.”

Traditional atomic clocks solve this problem by using the energy transitions of electrons within atoms as a stable reference. The Vienna team has now shown that the nucleus of a thorium atom can perform the same function.

The system works because thorium nuclei absorb laser light only when it has exactly the right frequency. If the laser frequency begins to drift, the amount of light absorbed by the nuclei decreases.

The system detects this change and automatically adjusts the laser back to the correct frequency. This continuous feedback keeps the clock running at a constant rate without requiring a traditional atomic clock to maintain its stability.

For the first time, scientists have demonstrated an independently functioning, self-regulating nuclear clock.

Nuclear clock accuracy: One second in 30 million years

The potential advantage of nuclear clocks comes from the extraordinary properties of atomic nuclei.

“The big advantage of Vienna’s new nuclear clock is that much higher precision is in principle possible using atomic nuclei instead of atoms,” says Torsten Schumm.

An atomic nucleus is more than 10,000 times smaller than the atom around it. Because of its extremely small size, it is less affected by external influences that can interfere with precise measurements.

This resistance to environmental disturbances makes nuclear transitions especially promising for ultra-high-precision timekeeping.

To evaluate the prototype, the researchers measured its performance over one day. The nuclear clock achieved a relative accuracy of approximately 10⁻¹⁵, equivalent to an error of about one second over 30 million years.

This figure represents the clock’s measured accuracy; it does not mean that scientists tested it continuously for 30 million years. Even so, the result demonstrates the remarkable performance already achieved by an early nuclear-clock prototype.

The new nuclear clock is not yet as accurate as the world’s leading optical atomic clocks, which remain the standard for high-performance timekeeping.

“Although this is not yet at the level of the world’s best optical atomic clocks, it is a great result for a first prototype,” Schumm says.

What comes next for nuclear timekeeping?

Researchers believe nuclear clocks have considerable room for improvement. More powerful lasers and higher-quality thorium crystals could significantly increase their precision.

Those advances could eventually allow nuclear clocks to outperform existing atomic clocks and establish a new standard for measuring time.

The potential impact extends beyond clockmaking. More accurate timekeeping can improve measurements of other physical quantities, giving scientists more sensitive tools for investigating the fundamental properties of nature.

The Vienna team’s work shows that a long-pursued scientific concept can operate as a self-sustaining timekeeping system. With the world’s first self-stabilizing nuclear clock now functioning independently, researchers have taken an important step toward measuring time with unprecedented precision.